Residual torque may be defined as the torque that remains on a threaded fastener after it has been tightened, and is typically measured by applying torque to the previously-tightened fastener and observing the behavior of the fastener. This is usually performed with a hand-operated torque wrench. The purpose of residual torque measurement may be to assess the performance of a power tool that previously fastened a given joint, or to simply determine whether the torque on a given joint is sufficient for its intended purpose. For example, an under-torqued fastener may vibrate or work loose. Conversely, if tension is too high, the fastener can snap or strip its threads.
Even with the advent of precise instrumented power tools, the need to measure or audit residual torque remains important for many reasons. First, there are dozens of sources of error that could cause an instrumented tool, one that initially indicated correct installation torque, to subsequently apply low actual joint torque. For example something as simple as a cracked socket can throw off targeted applied torque. In another example, a longer-than-normal extension used with a tightening tool may introduce error. The longer extension absorbs more rotational energy intended for the joint as compared to a shorter extension, thus lowering actual applied torque. Power tools in particular create variability by their very nature—high speed, constant motion, and high volume. As the gears in the right-angle drive of a power wrench collect dirt and wear, increasing friction absorbs torque and the sensor in the tool picks up less than accurate readings.
A second reason to precisely measure residual torque is the high cost of failure for many joints. Improper torque in safety-critical applications, such as steering gears or braking assemblies, can result in significant equipment damage, human injury, or even death. As recalls attributable to improper torque in the automotive industry and other industries continue, it would seem the need to resolve torque issues carries more weight than ever.
A number of torque measurement technologies and methodologies already exist, including measuring peak torque, detecting breakaway inflection points, and predetermining capture angles.
Assessing residual torque by means of a peak measurement strategy may be the oldest and most widely employed methodology today. Typically, peak-torque wrenches use a simple indicating dial to measure peak torque. In order for these devices to effectively and accurately measure residual torque, though, a significant amount of operator training and practice is required. Proper operation requires the operator to slowly and deliberately apply ever-increasing torque until the fastener just begins to move, and then release pressure. This slow approach is an attempt to reduce the amount of overshoot after the fastener starts to turn. For example, the torque-time curve of FIG. 1 depicts a peak torque of 55 Nm at approximately 1,250 milliseconds.
One such peak-torque device is described in U.S. Pat. No. 4,643,030 (Becker et al.), “Torque Measuring Apparatus”. Becker et al. discloses a torque wrench that includes a strain gauge in communication with a peak-hold circuit. The output of the strain gauge is held by a peak detector such that the maximum torque detected by the torque wrench is captured and displayed to a user.
The tendency to overshoot is central to many of the problems associated with using a simple peak-reading device, such as the one disclosed in Becker et al., to measure residual torque. Contributing to the problem are individual differences in human reaction time. An operator with quick reaction time tends to take lower readings than an operator with slower reaction time. Slower reaction time results in greater overshoot. In addition, since torque auditors typically take several hundred measurements in a shift, inconsistencies may creep into the process. Fatigue may cause a weaker pull on the wrench, or pressure to meet a schedule may lead to a quicker pull and greater overshoot. An example of overshoot is depicted in FIG. 2. In this example, an overshoot of only about 150 milliseconds resulted in a peak reading more than 10% higher than the torque applied at the start of fastener rotation.
While excessive overshoot creates false high readings using a peak-reading device, releasing the wrench before the fastener begins to turn causes false low readings. This all-too-common occurrence is usually triggered when a bump or vibration in the work piece is mistaken for fastener rotation. These false apparent indications of fastener rotation are more common when the work piece is in motion, as on an assembly line.
Even if it were possible to stabilize these sources of variance, the peak residual torque method remains inherently flawed. It measures torque at the point where the operator stops pulling on the wrench. This may occur before the fastener turns, shortly after the fastener turns, or significantly after the fastener turns. Lack of accuracy has a cost. Peak residual torque measurements are often so questionable that managers end up taking multiple measurements attempting to determine whether a torque problem really exists rather than taking corrective action with the fastening system.
Other known methods attempt to detect breakaway torque, or the torque required to overcome static friction, by looking for inflection points in the torque-time curve. These methods leverage the fact that resistance to the wrench changes at the point where static friction has been overcome. The torque-time curve of FIG. 3 depicts the capture of a torque-time breakaway point. In the hands of a skilled and very careful operator, torque-time breakaway detection can produce better quality measurements in less time than those taken using previously described peak measurement techniques.
For example, U.S. Pat. No. 4,426,887 (Reinholm et al.), “Method of Measuring Previously Applied Torque to a Fastener,” discloses a digital torque wrench and a method for detecting breakaway inflection points in a torque-time curve. The method looks for a breakaway inflection point by examining and storing progressively increasing torque values until detecting successive decreasing torque values. The point at which the torque values turn negative indicates a breakaway inflection point.
Though faster, torque-time breakaway inflection detection, such as that described in Reinholm et al., presents the user with challenges. Inflection points in the torque-time curve can easily be caused by operator hesitation, resulting in false low readings. Conversely, well-lubricated soft joints may produce very little or no detectable inflection at fastener motion. The torque-time curve of FIG. 4 depicts an example of a fastener with very little inflection in the torque-time curve at the start of fastener rotation.
The introduction of the solid state gyroscope has facilitated development of residual torque measurement devices that incorporate the use of sensed angular displacement as a qualifier for the capture of a torque value. One such method is referred to as the “capture angle” or “torque at angle” method, which captures torque at a predetermined degree of sensed angular rotation.
Using the capture-angle method, the residual torque for any given joint is the reading taken after some degree of sensed angular rotation past a torque threshold that includes both windup and actual fastener rotation. Windup, also known as flex, is understood to be the sensed angular motion due to the inherent metallic elasticity in the wrench, drive, extension, socket, fastener, and work piece. The amount of windup may vary considerably from joint to joint for a given assembly type, making it difficult to accurately determine an appropriate capture angle. This remains especially true for complex joint assemblies. For example) FIGS. 5a and 5b depict torque-angle curves for two different joints of a light truck assembly. In FIG. 5a, the amount of windup is less than one degree, yet for the same type of assembly, another joint demonstrates well over six degrees of windup.
With this variability in mind, to determine capture angle, typically, an engineer makes a best guess based on the materials used, their properties, the type of joint and anticipated windup before fastener rotation begins. Going forward, torque capture angle is often adjusted using some number of residual measurements and comparing them to in-line installation measurements. As such, the capture angle method tends to rely on trial-and-error techniques, and remains fairly subjective.
In one variation of the capture angle method, U.S. Pat. No. 6,698,298, “Torque Wrench for Further Tightening Inspection” (Tsuji et al.) discloses a gyroscope-based torque wrench and method of measuring torque. In Tsuji et al., the disclosed torque wrench measures torque and angle data, and combines the measured data with predetermined, referential torsion characteristics of the wrench and work piece. The method of Tsuji et al. stores into read-only memory a predefined reference torque-angle line that attempts to characterize the behavior of the wrench, including windup or flex, and relies on these predefined characteristics to extrapolate and estimate torque-angle slope intersections.
One problem with predefining wrench and work piece characteristics is that flex varies across wrench and work piece components, which creates variation in the windup slope from wrench to wrench and application to application. However, the method of Tsuji et al. assumes a constant, known characteristic and calculates slopes accordingly, in advance. This problem is exacerbated with high static friction joints where the slope of the torque-angle curve after restart is steepest. Furthermore, the method of Tsuji et al. remains highly affected by the non-rigidity, or softness, of the work piece.